The biology of neoplasia is complex and associated with: (1)\intrinsic aberrations of the neoplastic cell behavior; (2)\changes in the surface membrane of the tumor cell with presentation of new or rearranged molecules; (3)\the initiation of recognitive events directed to the tumor cell surface; and (4)\the character of the response of the host. Our research addresses selected issues in the biology of neoplasia, specifically: (1)\The structure, cellular function and molecular biology of candidate transformation-associated molecules. These currently include mammary tumor glycoprotein (MTGP), a new 126 kd phosphotyrosinated glycoprotein, and poorly characterized cell surface proteins with immunological homology to known oncogene sequences or containing phosphotyrosine. (2)\The genomic basis for expression of these molecules and relationship of expression to the transformed phenotype. (3)\The tumor antigen MTGP and others as potential targets for in vivo imaging of experimental breast carcinomas with monoclonal antibody synthesized by cloned hybridomas. (4)\The recognition of these molecules and the host immune response to it. (5)\Characterization and delineation of the genetic and functional restrictions in cellular cooperation required for immune induction of local activation of coagulation proteases and analysis of the bioregulatory properties of a discrete fibrinolytic peptide that is evolved, its effect on tumor cells and the immune attack against tumors. (6)\The molecular biology of receptor-mediated regulation of lymphoid function and the immune response by discrete serum lipoproteins and lipoprotein subsets to more clearly delineate an aspect of regulation of the host immune response to neoplasia.